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Coordination Chemistry Reviews xxx (2017) xxx–xxx

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Coordination Chemistry Reviews

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Review

Biological metal–organic frameworks: Structures, host–guest chemistryand bio-applications

https://doi.org/10.1016/j.ccr.2017.12.0030010-8545/� 2017 Elsevier B.V. All rights reserved.

⇑ Corresponding author.E-mail address: [email protected] (D. Li).

Please cite this article in press as: H. Cai et al., Biological metal–organic frameworks: Structures, host–guest chemistry and bio-applications, CoordRev. (2017), https://doi.org/10.1016/j.ccr.2017.12.003

Hong Cai a, Yong-Liang Huang b, Dan Li b,⇑a School of Chemistry and Environmental Engineering, Hanshan Normal University, Chaozhou, Guangdong 521041, PR ChinabCollege of Chemistry and Materials Science, Jinan University, Guangzhou, Guangdong 510632, PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 26 August 2017Received in revised form 2 December 2017Accepted 4 December 2017Available online xxxx

Keywords:Biological metal-organic framework(BioMOF)BiomoleculeHost-guest chemistryBio-application

Biological metal–organic frameworks (BioMOFs) are a new class of crystalline porous materials devel-oped in the last decade that represent a subclass of metal–organic frameworks (MOFs). Biomoleculesintroduced as components of MOFs confer biological compatibility for this emerging type of material,thus providing new opportunities for applications in biology, medicine, and a variety of other fields. Inthis review, to focus on host–guest chemistry and applications in biology and biochemistry, we providean overview of recent examples of BioMOFs comprising multifunctional biomolecular ligands and tran-sition metal ions. The bio-ligands include nucleobases, amino acids, polypeptides, proteins, cyclodextrin,porphyrin/metalloporphyrin and others. The host–guest chemistry of BioMOFs is highlighted in light ofsupramolecular recognition by different technologies. The potential applications of BioMOFs in severalpromising research fields such as drug delivery, enantioseparation and biomimetic catalysis are also sum-marized. In the last section, the outlook and possible challenges in advancing these research topics areillustrated.

� 2017 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 002. Design of BioMOFs with biomolecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

2.1. Nucleobases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 002.2. Amino acids, peptides and proteins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

2.2.1. Amino acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 002.2.2. Peptides. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 002.2.3. Proteins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

2.3. Porphyrins and metalloporphyrins. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 002.4. Cyclodextrin and other biomolecules. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

3. Host–guest chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
3.1. Thermodynamic and kinetic aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 003.2. Molecular optimization and simulation of interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 003.3. Structural and morphological details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
4. Potential bio-applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
4.1. Drug delivery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 004.2. Chiral separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 004.3. Biomimetic catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 004.4. Bioenvironmental protection materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 004.5. Biological motors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 004.6. Electrochemical sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
. Chem.

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2 H. Cai et al. / Coordination Chemistry Reviews xxx (2017) xxx–xxx

PR

5. Challenges and prospects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

1. Introduction

Metal–organic frameworks (MOFs) are an attractive class ofporous crystalline materials constructed of metal ions (or metalclusters) and organic ligands through coordination bonds intotwo-dimensional or three-dimensional extended periodic networkstructures [1,2]. These tailored porous materials have developedrapidly in the last two decades and show great promise for numer-ous applications, such as gas storage, separation, chemical sensing,catalysis, nonlinear optics, and potential biomedical applications[3–10]. Rapid developments in science and technology have pro-moted interdisciplinary fields and growth into a new research area,even attracting interest from industry sectors. During the discoveryof more than 20,000 MOFs, a new subclass of MOFs combiningMOF chemistry and bioscience has arisen, biological metal–organicframeworks (BioMOFs).

There is no clear definition of the terms related to BioMOFs. It isevident from the literature that there are two different views[11–15]. Some scientists believe that BioMOFs should be con-structed from at least one biomolecule serving as an organic ligand,whereas others consider BioMOFs a class of highly porous MOFsthat offer plenty of opportunities for applications across biologyand medicine. The former emphasizes the composition of MOFsas including biomimetic units derived from nature. The latterfocuses on applications in the biological domain. The requirementsfor bio-applications are much stricter than those for other types ofindustrial applications, and thus the toxicology, stability, efficacy,particle size and morphology of the BioMOFs must be comprehen-sively characterized. Regardless, BioMOFs combine crystalline por-ous materials and biological science. The study of BioMOFsencompasses not only the structural diversity and inner porosityof traditional MOF materials but also chirality, molecular recogni-tion and biomolecular catalytic function. Due to their low toxicityand good biocompatibility, BioMOFs have great potential applica-tions in biologically related fields. Although only studied for lessthan 10 years, BioMOFs have attracted wide attention because oftheir beautiful structures, rich supramolecular chemistry andunique biomimetic properties.

In this review, we will provide a summary of some designprinciples, host–guest chemistry and potential bio-applicationsfor BioMOFs using some biomolecules as multifunctional ligands.The biomolecular ligands include nucleobases, amino acids, pep-tides, proteins, cyclodextrins and porphyrins/metalloporphyrins(see Table 1). The structures and properties, supramolecular recog-nition as well as biomedical and biochemical applications such asbiomimetic catalysis, drug delivery and chiral separation arediscussed. We will also attempt to provide a new overview ofthe current state-of-the-art in this field. Most of the referenceswe cite were published during the last 5 years by Chinese scientistsas a contribution to the special issue of VIII Chinese CoordinationChemistry Conference. The outlook on host–guest chemistry andpossible challenges in advancing these research topics are alsodiscussed.

2. Design of BioMOFs with biomolecules

MOF performance and raw materials are meeting increasinglyhigher requirements with the rapid development of science andtechnology. BioMOFs are a new generation of MOFs that arebiocompatible materials that can reduce the impact on the

lease cite this article in press as: H. Cai et al., Biological metal–organic framewev. (2017), https://doi.org/10.1016/j.ccr.2017.12.003

environment. The incorporation of these biomolecules confersnew features such as molecular recognition, biosensing, biocataly-sis and self-assembly. In some situations, BioMOFs may generatenovel properties.

Some biological molecules have been successfully merged intoMOFs as organic ligands. However, symmetry defects make it moredifficult to synthesize ordered crystalline materials. In addition, theflexible structures of many biomolecules and labile metal–ligandcoordination facilitate interpenetration and unfavourable geome-tries or stoichiometries, resulting in nonporous frameworks. Thesedisadvantages hinder the development and application ofBioMOFs. Despite this, extensive research efforts have been aimedat synthesizing new BioMOFs. Some empirical methods can beused to obtain well-performing BioMOFs: (1) utilizing anasymmetric bio-ligand to prepare a highly symmetric secondarybuilding unit, such as bio-MOF-100 [16]; (2) introducing a highlysymmetric auxiliary ligand to compensate for the low symmetryof the bio-ligand, such as ZnBTCA [17]; (3) forming a cyclicoligomer by low-symmetry small biomolecules; (4) forming bio-molecules@MOFs by establishing a covalently coupled systemwithpre-existing MOF linkers or the external surface via postsyntheticmodification or by encapsulating biomolecules within the MOFpores via diffusion or permeation. This section highlights the latestadvances in the synthesis and structures of BioMOFs.

2.1. Nucleobases

Nucleobases are nitrogenous baseswith abundant self-assemblycharacteristics. The potential multiple coordination sites of thenucleobases together with their rigid structure lead to the forma-tion of accessible voids within the crystal structure, thus makingthem suitable for the design and synthesis of porous BioMOFs[18,19]. Nucleobases can be divided into two groups: purines andpyrimidines. The greater number of heteroatoms and hydrogenbonding donor/acceptor positions of purine nucleobases makethem more appropriate for providing coordination bonds or inter-acting with other entities than the pyrimidines [20]. According tothe Cambridge Structural Database (CSD), the number of guanine-containing MOFs is obviously lower than that of adenine-basedMOFs because of the poor solubility of guanine in common solvents.Here, adenine-based BioMOFs are mainly discussed.

The intrinsically low symmetry and limited length of adeninemolecules make it difficult to obtain 3D BioMOFs using a singleadenine ligand. An effective strategy is to introduce a high-symmetry auxiliary ligand with carboxylic acids to build metal–carboxylate–nucleobase BioMOFs [21]. Recently, our group [17]reported a new adenine-tagged BioMOF, namely ZnBTCA (BTC = benzene-1,3,5-tricarboxyl, A = adenine), as shown in Fig. 1. ZnII1and ZnII2 were triply bound by two bidentate carboxyl groupsand one adenine (N3 and N9 sites) to form a binuclear-Zn sec-ondary building unit (SBU), which was extended from the axialpositions via linking two monodentate carboxyl groups along thec axis. This fragment was further propagated through coordinationwith ZnII3 to construct the overall framework. The distinguishingfeature of ZnBTCA is the sinusoidal channels with open Watson–Crick sites periodically immobilized on the interior, which providesfavourable conditions for molecular recognition in host–guestchemistry.

Subsequently, utilizing isophthalic acid as an auxiliary ligand,we synthesized another new adenine-containing BioMOF ZnBDCA

orks: Structures, host–guest chemistry and bio-applications, Coord. Chem.


Table1

Summaryof

someBio-MOFs

andtheirpo

tentialap

plications

.

BioMOF

Biomolecules

Auxilia

ryliga

nd

Potential

application

Refs.

ZnBTC

AAde

nine

BTC

,ben

zene-1,3,5-tricarbo

xyl

Dru

grelease

[17,83

]BioMOF-10

0Ade

nine

4,40-B

iphen

yldicarbox

ylic

acid

Dru

grelease

[16]

ZnBDCA

Ade

nine

BDC,b

enze

ne-1,3-dicarbox

ylate

Molecularreco

gnitionan

dde

tection

[22]

PCN-530

Ade

nine

TATB

,4,4

0 ,400 -s-triazine-2,4,6-triyl-triben

zoate

Molecularreco

gnition

[23]

(Et 4N)[Cd 3

(ade

) 2(ipa

) 2Br]

3MeO

H�3.5DMA

Ade

nine

Isop

hthalic

acid

Sepa

ration

hyd

rocarbon

s[26]

(Et 4N)[Cd 3

(ade

) 2(thb)

2Cl]�3MeO

H�3.5DMA

Ade

nine

2,5-Th

ioph

ened

icarbo

xylicacid

[27]

Zn4(5-m

tz) 6( L-A

la) 2�2(D

MF)

L-Ala

=L-alan

ine

5-Hmtz,5

-methyltetraz

ole

Molecularreco

gnitionan

dsepa

ration

enan

tiom

ers

[29]

Zn4(5-m

tz) 6( D-A

la) 2�2(D

MF)

D-A

la=

D-alanine

Zn4(5-m

tz) 6( L-Ser) 2�2(D

MF)

L-Se

r=

L-serine

Zn4(5-m

tz) 6( L-V

al) 2�2(D

MF)

L-Val

=L-va

line

CuII(G

HG)

GHG=Gly-L-H

is-G

ly,

–Se

paration

forch

iral

drugs

[35]

bdh-Zn-T

122Hferritin

Ferritin

Ben

zene-1,4-dihyd

roxa

mic

acid

Enzy

matic

catalysis

[45]

15Fe

rritin–M

OFs

Ferritin

Ben

zeneh

ydroxa

mic

acid

[46]

[Zn16(H

2O) 8(M

nIII Cl-OCPP

) 4]

5,10

,15,20

-Tetrakis(3,5-biscarbo

xylphen

yl)porph

yrin

–Olefinsep

oxidationcatalysis

[71]

CZJ-6

5,10

,15,20

-Tetrakis-(3,5-dicarbo

xyph

enyl)porph

ine

–Biomim

etic

catalysis

[89]

MOF-12

01,

MOF-12

03L-lactate

Acetate

Food

safety

detection

[93]

PCN-222

TCPP

,Meso-tetra(4-carbox

yphen

yl)porph

yrin

–Ph

otoc

atalytic

water

trea

tmen

t,electroc

hem

ical

senso

r[94]

c-CD

MOF

c-CD,c

-Cyc

lode

xtrin

–Se

paration

ofch

iral

arom

atic

alco

hols

[87]

H. Cai et al. / Coordination Chemistry Reviews xxx (2017) xxx–xxx 3

Please cite this article in press as: H. Cai et al., Biological metal–organic framewRev. (2017), https://doi.org/10.1016/j.ccr.2017.12.003

with Zn4O core–shell structure via a solvothermal reaction(Fig. 2a). In the structure, four ZnII were quadruply bound by N3and N9 from two adenines, monodentate carboxyl groups ofbenzene-1,3-dicarboxylate (BDC) and the central oxygen atom,respectively, to form Zn4O(ade)4(COO)4 SBU. Then, every SBU wasextended by linking one of the monodentate carboxyl groups andN7 of imidazole. The other end of the carboxyl O was coordinatedwith the other independent ZnII to construct the overall frameworkwith large 1D channels (Fig. 2b). From the perspective of theoverall framework, ZnBDCA resembles a DNA helix connected bymetal ions along the b axis resulting in high helix stability(Fig. 2c). Interestingly, there is heterogeneity of the interactionbetween ZnBDCA and acriflavine similar to that of DNA [22]. It isexpected that ZnBDCA may have potential applications in biologi-cal detection and predetermined selectivity for given molecules.

Drawing on the synthesis strategy for bio-MOF-100, the Zhougroup [23] introduced a highly symmetric co-ligand, 4,40,400-s-triazine-2,4,6-triyl-tribenzoate (TATB), together with adenine toeffectively build a highly symmetric SBU that facilitatedthe packing of repetitive units in the formation of PCN-530,Zn3[Zn2(l2-H2O)]3(ade)6(TATB)4(DMF). The coordination modesof adenine retained open Watson-Crick sites for PCN-530, allowingthe incorporation of DNA base pair interactions into the BioMOFs.

In practice, another alternative strategy is to introduce paddle-wheel SBUs to construct BioMOFs. The adenine coordinates withmetal through the N3 and N9 positions to form a metal–adeninate‘‘paddle-wheel” subunit. Fei Wang et al. synthesized a new MOFconstructed from the cadmium–adeninate paddle-wheel units,[Cd2(ade)2(int)2(DMF) (H2O)]�DMF (int = isonicotinic acid), throughthe solvothermal method [24]. Moreover, inspired by the synthesisof bio-MOF-101 and bio-MOF-102, using asymmetric ligand 4-pyrazolecarboxylate (4-pca) and adenine, the Zhang group [25–27]synthesized two dynamic frameworks, [Zn5(4-pca)4(ade)2(H2O)2]�DMF�4H2O and [NH2(CH3)2][Zn3(4-Pca)3(ade)]�10DMF�8H2O, thatshowed reversible dynamic deformation with light hydrocarbons.The crystal structure of the latter BioMOF demonstrated flexibilityin compression–expansion. They subsequently introduced isoph-thalic acid (ipa) and 2,5-thiophenedicarboxylic acid (thb) to assem-ble with adenine and cadmium salt to obtain two isostructural andanionic porous metal–organic frameworks, (Et4N)[Cd3(ade)2(ipa)2-Br]�3MeOH�3.5DMA and (Et4N)[Cd3(ade)2(thb)2Cl]�3MeOH�3.5DMA, that exhibited high separation capacity for C2/C1 hydrocarbons.

In these BioMOFs, the N sites of adenine was not fully occupiedby metal ions, preserving the Watson–Crick sites and thus provid-ing possibilities to form hydrogen bonds with specific guests forthe further study of host–guest chemistry.

2.2. Amino acids, peptides and proteins

Amino acids, peptides and proteins are organic compounds con-taining ANH2 and ACOOH and are all excellent organic ligands forthe synthesis of BioMOFs. Two or more amino acids are dehydratedto form several peptide bonds and thus form a peptide. Manypeptides fold together to form a protein molecule (proteins areoccasionally referred to as polypeptides).

An amino acid is used as an example to introduce the coordina-tion modes of these biomolecules with metal ions. As shown inFig. 3, metal ions can coordinate with the amino-N atom and/orthe carboxyl-O atom. Carboxyl groups provide a variety of coordi-nation modes and have strong coordination ability with metal ionsdue to their large negative charge density. Carboxyl groups canalso form metal–carboxylic clusters or bridge with metal ions toincrease the rigidity and stability of the frameworks. Adjacentamino and carboxyl groups with special angles connect metal ionsin a certain direction to obtain unique extended network struc-



Fig. 2. (a) [(Zn4O) (ade)4(BDC)4Zn2] core–shell structure. (b) 1D channel of the ZnBDCA showing the pore. (c) Perspective view of the overall framework of ZnBDCA along baxis.

Fig. 1. (a) Structure of adenine labelled Watson-Crick face and Hoogsteen site. (b) The asymmetric unit of the ZnBTCA. (c) Framework fragments of ZnBTCA showing thecoordination geometries of the SBUs and BTC ligand as well as showing the Watson–Crick sites hanging in the framework. (d) Perspective view of the overall framework ofZnBTCA, showing the channel region (highlighted in yellow sphere) and inside view integrated with the Connolly surface (probe radius 1.0 Å), revealing the open Watson-Crick sites in the interior surface (highlighted in blue).


tures. Notably, amino and carboxyl groups can also be used as H-bond acceptors and receptors in the study of host–guest chemistry.

2.2.1. Amino acidsBioMOFs constructed purely from amino acids are rare. Most

are constructed by mixing amino acids or modified amino acidswith organic ligands. Xiao and co-workers [28] reported two novel3D chiral MOFs with chiral amino acid ligands containing n-fold


interwoven helices constructed from the mixed ligands 1,3,5-benzenetricarboxylic acid (BTC) and chiral histidine (His) and Zn(II). A typical m3-N1O1:O1:O2 coordination mode was obtained withHis and Zn(II) in the structure.

Using topological predictionmethods, the Zhang group [29] pre-pared a series of homochiral 3D BioMOFs with chiral amino acidsand achiral tetrazolate co-ligands, Zn4(5-mtz)6(L-Ala)2�2DMF (1L),Zn4(5-mtz)6(D-Ala)2�2DMF (1D), Zn4(5-mtz)6(L-Ser)2�2DMF (2L)



Fig. 3. The potential coordination models of amino acids and metal ions.

Fig. 4. (a) Scheme of the synthesis strategy of the homochiral MOFs with chiral amino acids; (b) the coordination environment of metal centres in 1L, 2L and 3L; (c) the 3Dframework of 1L with guest molecule DMF (ball and stick) in the pores.


Please cite this article in press as: H. Cai et al., Biological metal–organic frameworks: Structures, host–guest chemistry and bio-applications, Coord. Chem.Rev. (2017), https://doi.org/10.1016/j.ccr.2017.12.003



and Zn4(5-mtz)6(L-Val)2�2DMF (3L) (5-Hmtz = 5-methyltetrazole,

L-Ala = L-alanine, D-Ala = D-alanine, L-Ser = L-serine, L-Val = L-valine).The four compounds 1L, 1D, 2L and 2D are isostructural with ABWtopology and have a typical m3-N1O1:O2 coordination mode of theamino acids coordinated to the metal ions (Fig. 3). The asymmetricunit of 1L consists of two types of Zn centres as shown in Fig. 4b.One is tetrahedrally coordinated by three 5-mtz N atoms and onecarboxylate O of L-Ala. The other shows a distorted trigonal bipyra-midal configuration with 5-coordination. ZnII links with 5-mtzligands to form a layer and then is further linked to adjacent layersby L-Ala to form a 3D framework (Fig. 4c). Since the materials arehomochiral frameworks with large channels, the interactionsbetween 1L, 2L and 3L based on L-amino and D-carvone are strongerthan those based on L-carvone. Thus, the frameworks were appliedfor the chiral recognition and separation of D- or L-carvone.

The nature of the amino acid ligand determines the potentialchirality of this type of BioMOFs. In addition, the porosity of theframeworks has special applications for the recognition of somespecial molecules and selective separation, particularly forenantiomers.

2.2.2. PeptidesWith amino acids as substrates, thousands of species of pep-

tides can be synthesized by varying the type and order of aminoacids. They react with metal ions to form diverse 2D or 3Dpeptide-based BioMOFs. Many scientists have already madeendeavours in this area [30–35]. Dipeptides are the shortestpolypeptides and are often used for the preparation of peptide-based BioMOFs. The Rosseinsky group adopted carnosine dipeptide(b-alanyl-L-histidine, Car) as a ligand to react with Zn(II) ions toyield a ZIF-type 3D water-stable MOF, ZnCar, and then used Gly-Ser (GS) to form a layered porous framework Zn(GS)2 that exhib-ited adaptable porosity after guest removal [33]. To increase thepore metrics, the tripeptides Gly-L-His-Gly (GHG) and Gly-L-His-

L-Lys (GHK) were coordinated with CuII to form two isoreticular3D peptide-based porous BioMOFs (CuIIGHG and CuIIGHK). TakingCuIIGHG as an example, there are two different types of peptide-to-metal interactions: tridentate chelating with the His-X residue andmonodentate carboxylate bonding with the C-terminal Gly. Thebinuclear-Cu cluster acts as an SBU, and the sequence Cu-peptide-Cu defines fourfold helicoidal chains bridged into a 3Dframework via the formation of l2-carboxylate bridges (Fig. 5).Interestingly, the two BioMOFs reverse the original open channelstructure after exposure to water vapour, indicating sponge-likebehaviour [36].

Fig. 5. Structure of CuIIGHG frameworks. (a) The binuclear cluster SBU. (b) Perspective ofhave been omitted for clarity. Cu, dark blue; O, red; C, grey; N, blue; H, omit.


There are fewer polypeptide–MOFs than dipeptide–MOFs, likelybecause the flexibility of longer-chain polypeptides increases thedifficulty of forming 3D frameworks. However, it is the flexibilityof peptides that gives this type of BioMOFs adaptability anddynamic response characteristics for guest diffusion [37–39]. Fur-thermore, peptide-based BioMOFs also have potential applicationsas molecular switches [15,34,40,41].

2.2.3. ProteinsMany currently known natural proteins require binding of

metal ions at specific positions to ensure proper protein folding.However, it is difficult to control the coordination of metal ionsat the interfaces of proteins due to the complexity and flexibilityof protein structures. Therefore, obtaining 3D protein crystals bydesign is rarely successful. In 2014, the Jiang group presented anovel approach to fabricate protein crystalline frameworks (PCFs)[42–44]. They used an inducing ligand connecting the native pro-tein concanavalin A (ConA) via dual supramolecular interactionsto form PCFs. The inducing ligands, denoted RhnMan and RhnGlu,included three moieties: monosaccharide, rhodamine B (RhB) anda short oligo. The sugar species of a-D-mannopyranoside (Man)or a-D-glucopyranoside (Glu) was bound to ConA. The dimerizationof RhB connected the ligand-attached ConA by p–p stacking, and ashort oligo (ethylene oxide) tether of varying lengths was used tolink the sugar and RhB (Fig. 6) [42]. These three factors are veryimportant for synthesizing PCFs, and none is dispensable. Follow-ing this principle, ConA was replaced with they selected soybeanagglutinin (SBA) as a building block to successfully prepare 1Dhelical protein microtubes by combining molecular recognitionbetween lectin and sugars and p–p interactions [43]. Recently,they employed the native protein LecA with a cuboid shape asthe building block to obtain five types of protein assembly struc-tures, further confirming the feasibility of this strategy [44].

In addition, the Tezcan group designed a porous 3D proteincrystalline framework assembled by metal–organic linker-directed interactions. Interestingly, the ternary protein–metal–organic crystalline framework, bdh–Zn–T122Hferritin, was a highlyporous framework with a solvent content of 67% due to the hollowferritin hubs. The ferritin hubs retained their own nature and thuscould perform their enzymatic activity within the lattice. Moregenerally, this class of BioMOFs integrates protein building blocksand metal ions with organic linkers in a modular fashion [45]. Aftertwo years, they had systematically varied both the metal ions atthe vertices of the ferritin nodes (Zn2+, Ni2+, and Co2+) and the syn-thetic dihydroxamate linkers, ultimately yielding an expanded

CuIIGHG three-dimensional framework by l2-carboxylate bridged. Hydrogen atoms



Fig. 6. Crystal structure of ConA/Rh3Man. (a) The packing of assembly of ConA/Rh3Man. The four monomers in a tetramer: chain A (green), chain B (blue), chain C (purple)and chain D (yellow), and each tetramer was connected by Rh3Man dimers (shown in a stick model). The Rh3Man molecule bound to each of the four monomers of ConA,which are labelled as A0 , B0 , C0 and D0 . (b) and (c) showed electron density (r = 1.5) of the Rh3Man at the Man-binding cites of chain A, B, C and D, respectively.


library of 15 ferritin–MOFs. All of these studies indicate that thelattice symmetries and unit cell dimensions of protein–MOFs caneasily be varied in a modular fashion similar to the synthesis of tra-ditional MOFs, thus promoting the rapid development of crys-talline protein–MOFs [46]. Proteins, including antibodies andenzymes, are flexible 3D structures that have potential advantagesin constructing soft materials and biomimetic materials becausethey provide high chemical and structural diversity and possessinherent functions such as catalysis, electron transfer, and molecu-lar recognition [47].

2.3. Porphyrins and metalloporphyrins

Porphyrins are heterocyclic macrocycle compounds that con-tain four modified pyrrole subunits interconnected at their a-carbons via methane bridges [48,49]. There is escalating interestin constructing porphyrin/metalloporphyrin-based BioMOFs dueto their potential applications for gas storage, molecular machines,biomimetic catalysis and so on [50–60]. Tetrakis(4-carboxyphenyl)porphyrin (H4TCPP) is one of the most popular porphyrin ligandsfor constructing porphyrin-based BioMOFs. The Zhou groupreported several porphyrinic MOFs (named PCN-n) on the basisof a kinetically controlled synthetic process. Using a topology-guided strategy, they used the extended porphyrinic compoundsas linkers coordinated with ZrIV to form a series of Zr6 cluster-containing isoreticular porphyrinic-based BioMOFs, PCN-228,PCN-229, and PCN-230 [50–59]. The high connectivity of the Zr6cluster and the high charge density (Z/r) of ZrIV afford very strongMAO bonds with significant covalent character, and as a conse-quence, these BioMOFs were stable in aqueous solutions in a pHof 1–11. The use of metal ions, such as AlIII, FeIII, ZnII, CoII, and CdII,to construct nodes in different porphyrin/metalloporphyrin-carboxylate-based MOFs has also been extensively studied. The Zhougroup synthesized a series of mesoporous metalloporphyrin Fe-MOFs, namely PCN-600(M) (M = Mn, Fe, Co, Ni, Cu), as effectiveperoxidase mimics to catalyse the co-oxidation reaction. Wu andco-workers reported some porphyrinic MOFs based on pyridyl–or carboxy–porphyrin ligands and performed many forward-looking studies, including applications in catalysis and mimickingenzyme catalysis [61–70]. They used the metalloporphyrin MnIII-Cl–H8OCPP (H8OCPP = 5,10,15,20-tetrakis(3,5-biscarboxylphenyl)porphyrin) as a bridging ligand to connect paddle-wheel Zn2(COO)4SBUs, resulting in a porous BioMOF [Zn16(H2O)8(MnIIICl–OCPP)4]�solvents. This BioMOF showed high catalytic efficiency and selec-tivity for the epoxidation of small olefins. Based on the porosityof porphyrinic-based BioMOFs, the heterogeneous catalysis reac-tion mainly occurred inside the interior pore space of the BioMOFsand enhanced and sustained the catalytic activities [71].


2.4. Cyclodextrin and other biomolecules

Cyclodextrin (CD) is formed by cyclodextrin glucose residuetransferase (CGTase) from glucose polymers such as starch, glyco-gen and maltooligosaccharides, which are commonly classified asa-CD, b-CD and c-CD respectively. Because the outer edge ofcyclodextrin (Rim) is hydrophilic and the cavity is hydrophobic,it can provide a hydrophobic binding site. Zhang and co-workersused the microwave technique to rapidly and facilely (several min-utes) synthesize micron- and nanometre-sized c-cyclodextrinMOFs (c-CD-MOFs) for the first time. By premixing MeOH withthe size modulator PEG 20,000 during the modulation process,they obtained smaller crystals of 100–300 nm [72]. Lan et al. con-structed two extremely rare b-cyclodextrin (b-CD)-supportedMOFs through a template-induced approach (Fig. 7). The asymmet-ric unit of CD-MOF-1 contained only one CsI ion and one b-CDmolecule. The CsI ion was six-coordinated by six O atoms from fourcontiguous b-CD molecules, forming a strongly distorted pentago-nal pyramid geometry. The guest-accessible volume was attributedto the ‘‘malposition” stacking effect of the b-CD molecule. Theasymmetric unit of CD-MOF-2 contained one and one half crystal-lographically independent CsI ions and one b-CD molecule. The b-CD molecule in CD-MOF-2 concurrently bound with seven metalions, which, as vertexes, further assembled into a 3D BioMOF with1D channels. The filled ‘‘cages” endowed the eventual CD-MOF-2with large ‘‘gourd-shaped” cavities, resulting in an approximately25.3% guest-accessible volume. Both CD-MOFs can be used as bio-compatible drug carriers for the efficient delivery of anti-cancerdrug molecules [73].

In general, c-CD-MOFs rapidly disintegrate when exposed tohumid conditions. To overcome this poor stability, the Zhang groupdeveloped a straightforward one-step, effective strategy to enhancethe water stability of CD-MOF nanoparticles through surface modi-fication with cholesterol (CHS) to form a protective hydrophobiclayer [74]. These CD-MOFs maintained their cubic crystalline struc-tures even after 24 h of incubation, were well tolerated in vivo andincreased the blood half-life of DOX up to four-fold.

Cyclodextrins are an ideal host framework with characteristicssimilar to that of the enzyme model. Their ability to form inclusioncomplexes has been exploited in various areas of research (includ-ing MOFs) for drug carrier systems, the food industry, cosmeticsand more [75].

In addition to the above biomolecules, many biomolecules arenaturally present in humans, animals or plants. These are good nat-ural ligands that have been successfully integrated into MOFs toform functionalized BioMOFs with the properties of the biomole-cules themselves. The Zhu group linked curcumin to Zn2+ to obtaina highly porous BioMOF, medi-MOF-1, that possessed interesting



Fig. 7. Schematic view of structural forming of CD-MOF-1 (left) and CD-MOF-2 (right) by using different templates in the reaction process.


pharmacological properties in clinical studies. This BioMOF can beused not only as a carrier but also as a bioactive ingredient [76].

3. Host–guest chemistry

Host–guest chemistry is one of the defining concepts ofsupramolecular chemistry, which developed first in aqueous ororganic solutions. Cross-disciplinary research has promoted theapplication of the principles of supramolecular host–guest chem-istry in MOF materials. Due to the poor solubility of MOFs, studiesare mostly limited to the crystalline form. A two-phase solid–liquidcould provide a ‘real-life’ view of the assembly process. The heartof host–guest chemistry in MOFs is the concept of molecular recog-nition: mutually specific communication between the porous hostframework and the guest in definable structural relationshipsmainly occurring on the interior or surface of the solid. If the sizeand shape of the guest match with the pore of the MOFs, the guestcan be immobilized within the confined space of the host frame-work by hydrogen bonding, van der Waals forces, Coulombicforces, and even coordinate bonds. Indeed, most applications ofBioMOFs also rely on highly specific host–guest interactions. Atpresent, there are fewer examples of the exploration of host–guestchemistry in BioMOFs than in cyclodextrin, crown ethers and cal-ixarenes due to the short history of BioMOFs. The employment ofsome instrumental techniques and methods have greatly advancedBioMOF research. In the following sections, we provide some rep-resentative examples to highlight the study of host–guest chem-istry in BioMOFs using different technologies.

3.1. Thermodynamic and kinetic aspects

As mentioned above, most host–guest chemistry research hasbeen carried out in solution phase. Qualitative analyses and par-tially quantitative descriptions of thermodynamics and kineticsare well developed in supramolecular chemistry. By contrast, theseconcepts are still in their infancy in the BioMOF field due to the dif-ficulty of studying the solid state of crystalline MOF materials.Recently, our group reported the adsorptive performance of Bio-MOFs in single-component solutions and mixtures of a variety oforganic dyes with different ionic charges, dimensions, and contact-ing sites as complementary sets of control experiments to explorethe host–guest interactions in ZnBTCA with open Watson–Cricksites [17]. By comparing both BioMOFs, ZnBTCA and bio-MOF-1,we found that the hysteretic adsorptive behaviour of PfH+

appeared in ZnBTCA but not in bio-MOF-1. This differences is dueto spatial effects and host–guest interactions between the PfH+

dye and the openWatson–Crick site in ZnBTCA. We then used ther-modynamic and kinetic methodologies to interpret the unusual


hysteretic phenomenon. The calculated Arrhenius activationenergy for PfH+ uptake was 55.6 kJ mol�1, and the changes inenthalpy and entropy for the adsorption of PfH+ were 59.51 kJmol�1 and 197.62 J mol�1 K�1, respectively. These data reveal aunique energetic profile involving weak chemical bondingbetween the open Watson–Crick sites and the dyes bearing aminogroups.

There are various models, such as Langmuir, Freundlich,pseudo-first-order and pseudo-second-order. It is very importantto establish the optimization model in thermodynamic and kineticmethods. Different thermodynamic and kinetic parameters may beobtained by using different models. In addition, homogeneous/heterogeneous systems and solvation effects should be consideredwhen a model is chosen.

3.2. Molecular optimization and simulation of interactions

The equilibrium of guest uptake and recognition in a hostframework may be unfavourable for the formation of a periodicpattern of host–guest binding sites, hindering the acquisition ofprecise information on host–guest interactions. Theoretical calcu-lations and molecular modelling methods, including periodic den-sity functional theory (DFT), Monte Carlo (MC) and moleculardynamics (MD), can be powerful tools to optimize the possibleguest binding configuration within the constrained pockets of ahost framework to obtain a better understanding of the host–guestinteractions with molecular-level insights and forecast or explainthe experimental phenomena.

The Bei group used molecular simulations to study the adsorp-tion behaviour of ibuprofen in bio-MOF-1, bio-MOF-11 and bio-MOF-100. Ibuprofen molecules enter the narrow pores ofbioMOF-11 very slowly in rigid simulations and commonly diffusearound the metal ions clusters of BioMOFs [77]. Erucar et al. simu-lated the diffusion of ibuprofen, caffeine and urea in bio-MOF-100[78]. MD results showed that one H2O was close to the AOH ofibuprofen, whereas the other water molecules push the carboxy-late part of ibuprofen further apart (Fig. 8). These competing inter-actions between H2O molecules and ibuprofen decreased themobility of ibuprofen. The interactions between the guest mole-cules and metal atoms of MOFs were also analysed by radial distri-bution functions (RDF) because the metal sites in MOFs weregenerally the primary adsorption sites for guests.

Our group used grand canonical Monte Carlo (GCMC) and DFTsimulations to study adenine–thymine (A–T) base-pair bindingwithin the nanospace inside ZnBTCA [17]. The thymine moleculeswere distributed statistically around the Watson–Crick faces ofadenine and the benzene rings of BTC (Fig. 9c), thus indicated thatthe host–guest interactions could be mainly attributed to base



Fig. 9. (a) The thymine configuration was optimized by DFT calculations showing T-base interacting with A-base in ZnBTCA highlighting the host–guest fitting of shape andspecific binding sites. (b) The reverse Watson–Crick base pairing pattern in ZnBTCA pocket. (c) Calculated density distribution diagram of T-base in ZnBTCA-T by GCMC(yellow dots represent the statistical positions of T-bases).

Fig. 8. Ibuprofen adsorption in bio-MOF-100.


pairing and p–p stacking. The possible A–T binding configurationwithin the constrained pockets in ZnBTCA-T was optimized byDFT simulations, and reverse Watson–Crick base pairing wasobserved (Fig. 9a and b). This compelling evidence demonstratesthat the nature of the biomolecules was retained in this BioMOF,which may be a global rule. Therefore, this molecular simulationmethod can supplement and provide visual information for macro-scopic experiments.


3.3. Structural and morphological details

The advantage of pursuing host–guest chemistry in MOF mate-rials is that many MOFs have well-defined crystal structures withclear atomic, bond/interaction and geometry information, thusenabling the interactions of the guests to be envisioned. Structuraldetermination by the X-ray diffraction technique provides distinctsignatures for a better understanding of host–guest interactions.




The X-ray crystal diffraction technique allows the guest behavioursin crystalline porous MOFs to be determined more easily. Mostnotably, we can directly ‘see’ the structure of the host–guest com-plex combined with supramolecular interactions and obtain directinformation on the guest molecular configuration in the confinedspace, the distribution of the guest in the system, the interactionsites in the host framework and the interactive details among thehost–guest interactions. Mon and co-worker synthesized a water-stable 3D BioMOF, {CaIICuII

6[(S,S)-methox]3(OH)2(H2O)}�16H2O, fea-turing functional channels with thio-alkyl chains [79,80]. Indeed,the BioMOF showed a great affinity for AuIII and AuI salts in water,even in the presence of a wide variety of other metal cations typi-cally found in electronic wastes. The metal-selective captureoccurred via a metal ion recognition process, as demonstrated byexploring the host–guest chemistry on AuIII@BioMOF and AuI@-BioMOF. The X-ray single-crystal structures provided unprecedentedinsight into the S��Au interaction as well as the details of molecularrecognition. The guest Au(I) atoms in AuI@BioMOF were two-coordinated, anchored by S from the two thioether arms and addi-tional bridging Cl atoms in a quite linear geometry of the SAAuACltype. By contrast, the Au(III) atoms in AuIII@BioMOF were connectedto S from only one of the two thioether arms and further bound tothree Cl atoms in a distorted square planar coordination geometry(Fig. 10a and b).

Based on the accuracy of the information of host–guest interac-tions, the efficiency of the BioMOF in the capture of HgCl2 and CH3-HgCl salts from water was also investigated due to the well-knownaffinity of mercury for sulfur atoms. The BioMOF adsorbed bothmercury salts in a very fast, selective, and reversible manner. Thekinetic profiles of both adsorption processes were determined.The final loading of CH3HgCl was lower than that of HgCl2, likelydue to the host–guest methyl–methyl repulsive interactionsbetween the HgACH3 units and the SACH3 groups from methion-ine, which gave rise to a very close-packed structure with noempty space. The structures were clearly recognized by the

Fig. 10. (a) The single-crystal structures of {CaIICuII6[(S,S)-methox]3(OH)2(H2O)} after abs

3HgCl2@ BioMOF, (d) 5HgCl2@ BioMOF and (e) CH3Hg@ BioMOF clearly featuring highlydepending on the different chemical environments.


thioether arms of the methionine residues and confined into thechannels through S� � �Hg interactions (Fig. 10c–e).

In addition to X-ray diffraction technology, scanning tunnellingmicroscopy (STM) and transmission electron microscopy (TEM)can also be applied to host guest chemistry. High-resolution STMand TEM allow dynamic aspects on the solid–liquid surface to befollowed in these systems, and molecular recognition events canbe captured in real time [40]. Matsui et al. observed that dipheny-lalanine (DPA) peptides were assembled into a single crystallinestructure on the MOFs/water interface by TEM, and the electrondiffraction patterns showed the reorganization of DPAs on theinterface (see detail in Section 4.5. Biological motors) [82]. In situNMR and in situ Raman/IR spectroscopy are also powerful tech-niques that can be used as characterization tools to provide specificinformation about host–guest interactions. Studying host–guestchemistry in BioMOFs with different techniques has a great advan-tage over other amorphous systems for helping scientists deeplyunderstand the host–guest relationship for the further design ofMOF materials with specific functions to meet new needs. How-ever, many problems remain to be comprehensively investigated,such as host selectivity, binding regioselectivity, driving forceand binding mechanisms. Researchers are increasingly dedicatingefforts to the study of host–guest chemistry in BioMOFs throughexperimental studies assisted by theoretical calculations. Forexample, Martí-Gastaldo et al. simulated the adsorption of thetwo enantiomers of ephedrine ((+,�)-EP) in Cu(GHG) by MC simu-lations and found that Cu(GHG) displayed stereoselectivity for(+,�)-EP. Thus, they used Cu(GHG) as a chiral solid phase to isolatemore than 50% of (+)-EP from a racemic mixture using anexperimental method [35]. Wang et al. incorporated a cationicdye within bio-MOF-1 to enhance the detection and discriminationof nitro-explosives based on the host–guest properties of thedye@bio-MOF-1 composite. The fluorescence quenching effectwas attributed to electron transfer and energy transfer betweenthe dye@bio-MOF-1 and trinitrophenol (TNP) based on nonlinear

orbing AuCl and (b) AuCl3. (c) Lattice asymmetric units without solvent molecules offlexible arms, which show different conformations of the ethylenethiomethyl chains




Stern–Volmer plots and time-dependent fluorescence and absorp-tion spectra. Each explosive molecule was optimized by DTF. Thevalues of the HOMO–LUMO energies clearly explained the mostdramatic quenching effect of the dye@bio-MOF-1 for TNP [81].

4. Potential bio-applications

As mentioned above, BioMOFs are a subclass of MOFs and notonly have valuable applications in gas storage and separation, sen-sors, and catalysis but also have several promising applications inrelated areas of biology, such as drug delivery, enantioseparation,biomimetic catalysis, biological motors and environmental protec-tion materials. Although the bio-applications of BioMOFs have notbe deeply explored due to their short history, in this section, weillustrate somepotential applicationswith representative examples.

4.1. Drug delivery

To overcome the intrinsic limitations of therapeutic drugs andrealize controllable drug delivery and target delivery, great effortshave focused on the development of drug carriers. There are somebasic requirements for a drug carrier. One of the most importantconsiderations is that the drug must be released at a certain rateuntil reaching the target to achieve an appropriate dose within agiven period. BioMOFs are of interest as drug delivery hosts mainlydue to their high drug loading capability and excellent biodegrad-ability. Recently, we reported the use of BioMOF ZnBTCA as aself-sacrificing carrier to facilitate the uptake and release of theanti-cancer drug dinuclear gold(I) pyrrolidinedithiocarbamatocomplex, which exhibits potent in vitro cytotoxic activitiestowards A2780cis cisplatin-resistant ovarian cancer cells. Asshown in Fig. 11, ZnBTCA did not exert significant cytotoxicityagainst the cancer cells, and the drug was released in a sustainedmanner from drug@ZnBTCA to inhibit the cellular growth ofA2780cis cells. Compared to the free drug, drug@ZnBTCA reducedthe initial high cytotoxicity. Moreover, both the free drug anddrug@ZnBTCA inhibited the migration of A2780cis cells effectively,whereas ZnBTCA itself did not. The biocompatible ZnBTCA was a

Fig. 11. Transwell assay-based cytotoxicity and antimigration studies of A2780ciscells: (a) schematic representation of the transwell studies; (b) 72 h cytotoxicprofiles of the cells with different co-incubation times with ZnBTCA anddrug@ZnBTCA; time-dependent cytotoxicity of drug at 8.7 mm (naked drug) isshown as a reference.


good self-sacrificing carrier for sustained release of the anti-cancer drug upon degradation of ZnBTCA in water [83]. Inaddition, An et al. utilized bio-MOF-1, bioMOF-4, bioMOF-100,and bioMOF-102 as delivery materials to load etilefrine hydrochlo-ride to investigate the drug release behaviour through cationexchange. Each BioMOF exhibited an initial burst of drug release,followed by a gradual release of the remaining drug moleculesfor a long time [84].

The Qian group selected the porphyrin-based MOF PCN-221 asan oral drug vehicle due to its structural stability and methotrexate(MTX) as the model drug molecule due to its low tolerated dose.The drug delivery behaviour of MTX-loaded PCN-221 was testedunder various physiological environments. PCN-221 has two typesof nanoscale cages, which are larger than the minimum size of theMTXmolecule. MTX in the larger cages was released first in the ini-tial stage due to the weak bonding between the framework and thedrugs; a few drug molecules located near the walls of the smallercages were subsequently released slowly after 72 h because theaperture was smaller than the maximum size of MTX. Therefore,PCN-221 is a promising oral delivery carrier that shows high drugloading and sustained release behaviour without a ‘‘burst effect”[85]. Selecting pre-existing high-stability MOFs to load drugs isanother important strategy; the cargo-loaded MOFs can then befurther capped through binding of b-cyclodextrin (b-CD) on theMOF surface, which serves as a gate keeper to prevent thepremature release of drug [73–75]. Most recently, the Lan grouputilized two b-CD-based BioMOFs, Cs(OH)�(C42H70O35) and[Cs1.5(C42H66.5O35)]2, as carriers, and fluorouracil and methotrexatewere employed as model drug molecules to perform controlleddrug delivery and cytotoxicity assays [73]. The two CD-basedBioMOFs exhibited favourable potential as effective drug carriers.

In brief, drug delivery is the most likely application of BioMOFsystems. Numerous studies have shown that BioMOFs are excellentplatforms for drug delivery, whether for targeted delivery orsustained release.

4.2. Chiral separation

Enantiomers can be profoundly different by exhibiting exactlyopposite biological activities, pharmacologies and toxicities. Chiralresolution is a process for the separation of racemic compoundsinto their enantiomers. Chiral separation is an important andchallenging topic, especially in the field of biomedicine. Yan andco-workers adopted a c-CD MOF with a hydrophobic cavity andhydrophilic rim as the stationary phase of HPLC to efficientlyseparate twelve chiral aromatic alcohols. All R-enantiomers ofthese twelve chiral aromatic alcohols were eluted before the S-enantiomers, demonstrating that S-enantiomers have greater affin-ity for the c-CD MOF than their R-enantiomers. The c-CD MOF pro-vided higher resolution in the separation of (R,S)-1-phenyl-1-butanol, (R,S)-a-vinylbenzyl alcohol, (R,S)-1-(1-naph-thyl) ethanoland (R,S)-1-phenyl-1-pentanol than the CHIR-ALPAK IA commer-cial HPLC column and better resolution of (R,S)-1-phenyl-1-butanol, (R,S)-a-vinyl-benzyl alcohol, and (R,S)-1-phenyl-1-pentanol than the CHIR-ALPAK IB commercial HPLC column. Thegood performance of c-CD MOF mainly depends on hydrophobicand H-bond interactions between aromatic alcohols and c-CDMOF [86,87].

Interestingly, amino acids, oligopeptides, etc. are also well sui-ted for the production of functional chiral MOFs. Martí-Gastaldoand co-workers used a chiral 3D BioMOF, CuII(GHG), to separatethe chiral drugs methamphetamine (MA) and ephedrine (EP)[35]. The surface of the CuII(GHG) empty space was decorated withcarboxylate, amide, amino and imidazole groups towards theinside of the channels to provide abundant interaction sites forthe EP enantiomers. The adsorption of (�)-EP was solely directed




by the formation of a weak elongated H-bond (2.36 Å) with theamide bond in the C-terminal Gly of Cu(GHG). By contrast, (+)-EPformed short His-EP H-bonds (1.89 Å) (Fig. 12). The synergeticstrength of the H-bonds translated into a larger difference in theadsorption energies, allowing Cu(GHG) to act as a chiral solidphase extraction cartridge that permitted the isolation of morethan 50% of (+)-EP from a racemic mixture in only four minutes.Therefore, Cu(GHG) has potential applications in stereoselectiveseparation, especially for EP enantiomers. In addition, Zhanget al. reported six homochiral BioMOFs with amino acid ligandsas the chiral stationary phases for HPLC separation of enantiomerssuch as 1,2-diphenyl-1,2-ethanediol, pindolol, and furoin.[Zn(L-Tyr)]n(L-TyrZn) (Tyr = tryptophan) and [Co(L-Glu)(H2O)�H2O]1 exhibited better chiral recognition ability than the otherfour BioMOFs based on a comparison of separation effects [88].Chiral BioMOFs have broad prospects in the separation of race-mates, including chiral drugs.

4.3. Biomimetic catalysis

Enzymes are powerful macromolecular biocatalysts that canaccelerate chemical reactions with incomparable efficiency andspecific selectivity under mild conditions. Therefore, biomimetic

Fig. 13. (a) Perspective view of the crystal structure of CZJ-6 along the a axis. (b) A portiocages (the balls with different colors indicate different cages). (c) The Scheme 1 of propmetalloporphyrin and NHPI catalyst platform.

Fig. 12. Peptide–BioMOF for enantiosele


catalysis is a very attractive area with great prospects for highlyefficient catalysis. Constructing functional systems to mimic natu-ral enzymes is a highly sought-after goal. In particular, metallopor-phyrins and their derivatives, which act as the catalytic centres ofsome enzyme families, have attracted continuing attention.Recently, the Wu group [71,89–91] utilized a porous metalloporphyrin–BioMOF, CZJ-6 ([Cu4(H2O)4(Cu-L)]�4DMF�7[(CH3)2NH2]NO3�26H2O (L = 5,10,15,20-tetrakis-(3,5-dicarboxyphenyl)porphine)), to mimic enzymatic catalysis. The aerobic oxidation ofethylbenzene was chosen as a model because this reaction iseasily realized by enzymes under mild conditions.N-hydroxyphthalimide (NHPI) and the sandwich-type polyox-ometallate ({Zn3Co2W19}) were introduced into CZJ-6 to form atri-component biomimetic catalyst platform that greatly enhancedthe oxidation reaction with remarkable selectivity. The hydrophilicand hydrophobic nano-cages of CZJ-6 sufficiently large to accom-modate the substrates and co-catalysts and facilitated the recogni-tion of the substrate and product molecules. Moreover, theporphyrin–CuII sites readily accessed the substrate. The combinedcatalyst system exhibited enzyme-like features with very high effi-ciency, selectivity and sustainability to realize highly efficient bio-mimetic activation of molecular oxygen under mild conditions(Fig. 13) [89,90].

n structure of CZJ-6, showing the substrate-accessible porphyrin–CuII sites and poreosed catalytic cycles for the aerobic oxidation of ethylbenzene by the biomimetic

ctive separation of (+,�) ephedrine.




In addition, M–TCPP(Fe) (M = Co, Cu, and Zn), a series of ultra-thin 2D iron–porphyrin nanosheet-based films, was assembledinto multilayer films on electrodes through the Langmuir–Schäfermethod. These films were used as electrochemical platforms todetect H2O2 based on the heme-like activity of the nanosheetsM–TCPP(Fe). They exhibited the well-defined reduction peak asso-ciated with the reduction of H2O2, especially the GC/(Co–TCPP(Fe))n electrodes. The thickness of the BioMOF film greatly influ-enced the catalytic activity. The GC/(Co–TCPP(Fe))5 electrodeexhibited higher sensitivity, excellent stability and reproducibilityand thus can be used as a H2O2 biosensor for the real-time trackingof H2O2 secreted by live cells [92]. In addition, PCN-22x, includingPCN-221, 222, 224, and 225, have certain catalytic functions. Inparticular, PCN-222(Fe) possesses excellent catalase-like catalyticactivity [50–53].

4.4. Bioenvironmental protection materials

The interdisciplinary integration of MOFs with environmentalsciences, food chemistry, nanotechnology etc., could yield manypotential applications, such as in the agricultural industry, foodsafety detection and environmental improvement. Recently, theYaghi group reported two porous chiral MOFs using L-lactate andcalcium ion, Ca14(L-lactate)20(acetate)8(C2H5OH)(H2O) (MOF-1201) and Ca6(L-lactate)3(acetate)9(H2O) (MOF-1203), capable ofencapsulating cis-1,3-dichloroporopropene, an agriculturallyimportant fumigant, for slow release. These carrier materials werenaturally degradable in water, decreasing their environmentalimpact. These types of BioMOFs represent eco-friendly materialsthat can expand the scope of application [93]. In addition, Guet al. employed a zirconium-porphyrin BioMOF (PCN-222) as a cat-alyst to remove and photodegrade bisphenol A (BPA). The catalysisoccurred inside the MOF pores rather than in the solution phase.The saturation adsorption capacity was as high as 487.69 ± 8.37mg g�1, and the highest removal efficiency was 99.3% in the pHrange of 6–10. The Zr–porphyrin MOFs have excellent reusabilityand a wide practical pH range and thus could be applied to photo-catalytic water treatment processes [60,94].

Allyl isothiocyanate (AITC) is a potent natural antimicrobialcompound. Lashkari et al. used HKUST-1, RPM6-Zn, and MOF-74(Zn) as novel delivery systems for the encapsulation and controlledrelease of volatile AITC molecules for food preservation technology.The volatile AITC molecules diffused into the food matrices slowlywithout direct contact with food based on structural damage to theMOFs triggered by moisture. As carriers, BioMOFs have potentialapplications for food safety and food industry applications [95].

4.5. Biological motors

Chemical energy can be efficiently converted to mechanicalmotion by chemical reactions or reorganization of molecules bymeans of the Marangoni effect. Many scientists have stored organicmolecules with gels and released them to trigger motion. However,the energy conversion efficiency was quite low because theseorganic molecules were discharged in random directions. Basedon the reorganization of self-assembled peptides to generate aniso-tropic surface tension gradient at an interface, the Matsui groupadopted MOFs for DPA peptide storage and developed biochemicalmotors [82]. The guest DPA peptide was released from the MOFs byadding Na-EDTA aqueous solution, followed by reconfiguration ofthe peptide at the water/MOFs interface. The resultant creationof a large surface-tension gradient around the MOFs caused theMOF particles to move in the direction of higher surfacetension. Taking advantage of the nature of peptides, the use of


peptide@MOFs composites as autonomous biological motors delin-eates a new application of BioMOFs in biological machinery withpotential for further expansion.

4.6. Electrochemical sensor

The initial motives for incorporating biomolecules in MOFs maytake their inherent properties into account. The porphyrin ring isaromatic, with a total of 26 electrons in the conjugated system.Electron transfer can occur under certain condition. Based on thisfeature, the Zhang group reported Zr–porphyrin MOFs (PCN-222)as a simple and rapid photoelectrochemical sensor for the label-free detection of a phosphoprotein (a-casein). PCN-222 exhibitedan enhanced photocurrent response toward dopamine in O2-saturated aqueous media. In addition, CuII-metallated 3D BioMOF{CuTCPP[AlOH]2}n (referred to as PAC) successfully combined theadvantages of MOFs and nanomaterials for use as sensitive fluores-cence probes for selective H2S detection at physiological pH [96].

5. Challenges and prospects

Many biomolecules have unique features in nature, such as dis-coloration, luminescence, recognition, self-adaptation and self-healing. Nature can provide new insights for the design of newsmart materials by drawing on the special structures and proper-ties of biomolecules. Various biomolecules or biomimetic unitshave been introduced into MOFs as building blocks with the aimof conferring on the crystalline porous MOFs the analogous func-tions of biomolecules. Successful examples include imparting theflexibility of peptides onto crystalline porous systems for adapt-ability and the incorporation of proteins or enzyme into MOFsfor biomimetic catalysis.

Although significant progress has been made in the fabricationof BioMOFs, several challenges with bio-applications still remain.(1) The synthetic guiding protocol of global topological predictionis difficult to apply for the synthesis of BioMOFs due to the lowsymmetry of bioligands, which represent sources of unpredictabil-ity in the self-assembly process. Moreover, it is difficult to ensurethat the open active site and porosity of BioMOFs will be retainedin parallel in the resultant products. (2) The thermostability andchemical stability of BioMOFs do not yet meet the requirementsof traditional biomaterials. For example, the wide existence in bio-logical environments of phosphate, which exhibits a strong affinityfor metal clusters of MOFs, would rapidly destroy the coordinationequilibrium between metal nodes and ligands toward the forma-tion of metal–phosphate. Improving the stability and cell uptakeof BioMOFs are the main problems restricting their biologicalapplication. (3) In terms of bio-application, many practical prob-lems of BioMOF materials warrant serious consideration, such astoxicity, morphology, machinability, dissolvability, and the prepa-ration of homogeneous nanocrystals. Their biosafety, bio-distribution and efficacy in vitro and/or in vivo must approachthe requirements of real biomedical applications.

The judicious selection of biomimetic units based on a consider-ation of their charges, geometries, and bio-active sites is key toengineering BioMOF architectures. The accumulation of a molecu-lar bank of BioMOFs will provide more candidates for bio-relatedapplications. We believe that BioMOFs, especially those with openactive sites, have a promising future.

The special functional groups of bio-ligands and abundantinteraction sites with the guest make BioMOFs excellent matricesfor enzyme immobilization, molecular switches and biologicalmotors. Furthermore, many biomolecules have special properties,such as electron transfer and specific recognition abilities. The nat-




ure of these bioligands will endow BioMOFs with special propertiesand provide a foundation for the design of materials with potentialapplications in bioelectrochemistry, bionanotechnology, nanome-dicine and material sciences. Thus, we anticipate that BioMOFs willbecome a superstar in the field of smart materials.

Acknowledgements

This work was financially supported by the National BasicResearch Program of China (No. 2013CB834803), the NationalNatural Science Foundation of China (Nos. 21731002, 91222202and 21701038) and Scientific Research Start-up Funds of HanshanNormal University (QD20161009).

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